Phosphate glasses for radioactive, hazardous and mixed waste immobilization

ABSTRACT

Lead-free phosphate glass compositions are provided which can be used to immobilize low level and/or high level radioactive wastes in monolithic waste forms. The glass composition may also be used without waste contained therein. Lead-free phosphate glass compositions prepared at about 900° C. include mixtures from about 1 mole % to about 6 mole %.iron (III) oxide, from about 1 mole % to about 6 mole % aluminum oxide, from about 15 mole % to about 20 mole % sodium oxide or potassium oxide, and from about 30 mole % to about 60 mole % phosphate. The invention also provides phosphate, lead-free glass ceramic glass compositions which are prepared from about 400° C. to about 450° C. and which includes from about 3 mole % to about 6 mole % sodium oxide, from about 20 mole % to about 50 mole % tin oxide, from about 30 mole % to about 70 mole % phosphate, from about 3 mole % to about 6 mole % aluminum oxide, from about 3 mole % to about 8 mole % silicon oxide, from about 0.5 mole % to about 2 mole % iron (III) oxide and from about 3 mole % to about 6 mole % potassium oxide. Method of making lead-free phosphate glasses are also provided.

This invention was made with Government support under contract numberDE-AC02-76CH00016, between the U.S. Department of Energy and AssociatedUniversity, Inc. The Government has certain rights in the invention.

This is a divisional of application Ser. No. 08/772,982 filed Dec. 23,1996, now U.S. Pat. No. 5,840,638.

BACKGROUND OF THE INVENTION

The present invention relates to low temperature, lead-free, phosphateglass compositions which can be used to encapsulate and immobilizeradioactive, hazardous and mixed wastes. Specifically, the presentinvention concerns the development of novel phosphate glass compositionshaving reduced processing temperatures and improved chemical durabilitywhen compared with existing conventional borosilicate and phosphateglass formulations. The new materials of the present invention have beendeveloped primarily for use in vitrifying radioactive, hazardous andmixed wastes, but can also be used in other industrial glassapplications.

Industrial, utility, military nuclear operations produce radioactivehazardous and mixed wastes which must be treated in an appropriatemanner before disposal to prevent environmental contamination.

Vitrification has been selected for immobilization of high-levelradioactive waste (HLW) because glass is highly stable, very durable andhas the ability to incorporate a wide variety of chemical contaminants.However, requirements for storage and disposal of low-level radioactive(LLW) and mixed wastes differ from those for HLW. In addition, LLW andmixed waste contaminants have waste chemistries which can vary fromthose typically found in HLW. Thus, evaluation of vitrification orimmobilization of these wastes must consider these differences.

In the past, most research on vitrification of mixed waste has focusedon borosilicate glass formulations originally developed for HLW.Although borosilicate glass has demonstrated good long-term chemicaldurability and is both thermally and physically stable, it requiresrelatively high process temperatures (1200°-1500° C.) for effectiveencapsulation of waste. These high temperatures are a major drawbackbecause of volatilization of certain isotopes such as ⁹⁹ Tc and ¹³⁷ Csand heavy metals, such as Pb and Cd. To capture and stabilize theoff-gas contaminants high temperature processes require the use ofsecondary treatment systems. Further, borosilicate glass processing isincompatible with even minor amounts (>1.5 mole %) of P₂ O₅ in a wastestream. Insoluble phosphate phases form depending on the amount of CaOand rare earth oxides present. Further, wastes containing more thanabout 0.3 mole % Cr₂ O₃, more than about 16 mole % Al₂ O₃, or more thanabout 19 mole % Fe₂ O₃ have also been flagged as outside theborosilicate glass concentration envelope. Therefore, alternativevitrification processes or products are desirable for improved treatmentof these wastes.

In recent years, most research on low temperature glass processesfocused on phosphate glass. Phosphate glasses are network-formingmaterials, structurally similar to silicate glasses, whose basicbuilding blocks are PO₄ tetrahedra. Compared with silicate glasses,phosphate glasses offer significant advantages for waste vitrification,such as lower melting and softening temperatures and low meltviscosities. Use of phosphate glasses as final waste forms has beenpreviously proposed, however interest in their development waned asdurability of the glass matrix was found to be poor when compared withsilicate glasses. Further, new process technologies and materials wouldhave been required to handle their production.

Early work to develop phosphate glass as a nuclear waste form waspioneered at Brookhaven National Laboratory (BNL) under contract withthe U.S. Atomic Energy Commission, beginning in the late 1950's andcontinuing through the late 1960's as described by Tuthill, et al. in"Development of the Phosphate Glass Process for Ultimate Disposal ofHigh-Level Radioactive Wastes," BNL 50130(T-505) (1968). Work at BNLfocused on development of a continuous glass-melting process for thetreatment of Purex-type wastes, nitric acid solutions of the fissionproducts and residual salts from the extraction of uranium andplutonium, together with corrosion products. Development of the BNLprocess was carried out through pilot-scale testing using simulatedwaste solutions. A plant-scale demonstration using actual Purex HLW wasconducted during the mid to late 1960's as part of the WasteSolidification Engineering Prototypes (WSEP) program at the PacificNorthwest Laboratory (PNL) as described by McElroy, et al., in"Evaluation of WSEP High-Level Waste Solidification Processes" WasteSolidification Program Summary Report, Vol. 11, BNWL-1667 (1972).

In the BNL process aqueous Purex wastes were evaporated, formingprimarily nitrate salts of Fe, Na, Al, Cr. and Ni, SiO₂, SO₄, and PO₄anions were also present in the waste, along with fission products,nominally in the concentration range of 10⁻³ M. Good glass formation wasreportedly observed at P₂ O₅ compositions greater than 65 mole %, forthe Fe₂ O₃ --Na₂ O--P₂ O₅ ternary system, and at greater than 70 mol %for the Fe₂ O₃ --Al₂ O₃ --Na₂ O--P₂ O₅ quaternary system. However, theFe₂ O₃ --Na₂ O--P₂ O₅ system, all glasses with less than 60 mole % P₂ O₅devitrified. For the Fe₂ O₃ --Al₂ O₃ --Na₂ O--P₂ O₅ system, all glasseswith less than 70 mole % P₂ O₅ devitrified.

Glass formulations were melted at 1100°-1200° C., with temperaturesreduced to 800°-900° C. during idling periods to prevent volatilizationof phosphate. Metal ions were typically retained at more than 99.9% oftheir original amounts. Exceptions were ruthenium (94-99%) and cerium(99%), which appeared in the off-gas condensate, which was mainly H₂SO₄. The melt vessel was constructed of Inconel 702 with a Pt liner.Corrosion of the Pt liner was not a problem, although cracks did developon several occasions, corroding the container. Inconel in contact withthe molten phosphate reacted to form elemental phosphorus which, inturn, combined with Pt to form alloys with lower melting points.

In Europe, in another example of a waste solidification process,phosphate glasses were investigated for solidification and metalembedding of liquid radioactive wastes as described by Van Geel, et al.in "Solidification of High Level Liquid Waste to Phosphate Glass-MetalMatrix Blocks," IAEA-SM-207/83, Vol. 1, pp. 341-359, (1976). Glass wasmelted at about. 1100° C. using a refractory lined melter with a liquidwaste feed. Glasses were prepared as small beads to avoiddevitrification problems. Low-alkali or alkali-free iron-aluminumphosphate glasses were found to have improved chemical durability, evenbetter than that of borosilicate glasses. Soxhlet (dynamic)leachabilities were found to be about 5×10⁻⁵ g/cm² /d, although attemperatures above 100° C., leachability increased rapidly. Glasses werenominally 5 wt % Al₂ O₃, 15 wt % Fe₂ O₃, and 50 wt % P₂ O₅, containingabout 30 wt % waste oxides.

Lead-iron phosphate (LIP) glass nuclear waste forms were introduced andcharacterized in detail by Sales and Boatner in the mid 1980's. Whilelead phosphate glasses had been studied since the mid 1950's, their poordurability in water made them of little commercial interest. Vitrifiedwaste forms containing iron oxide, however, were found to fare favorablycompared with borosilicate glass waste forms, rekindling new interest inphosphate glass waste materials. LIP glasses could be processed attemperatures 100° C.-250° C. lower than borosilicate glasses using glassmelting technology similar to that developed for borosilicate glasses.Solidified forms had dissolution rates in water about 1000 times lowerthan comparable borosilicate formulations, at 90° C. in solutions with apH between 5 and 9.

The addition of iron to lead phosphate glass was found to dramaticallyincrease chemical durability of the glass, by a factor of about 10⁴ fora 9 wt % iron oxide addition. Also, the tendency for the glass tocrystallize on cooling or reheating was greatly suppressed. Without theiron modifier, lead metaphosphate glasses completely crystallized in airat 300° C. within a few hours. In contrast, LIP glasses were heated forseveral hundred hours at 500° C. without any signs of devitrification. Avariety of metal oxide modifiers were investigated for similar effectsMgO, Al₂ O₃, CaO, Sc₂ O₃, TiO₂, VO₂, Cr₂ O₃, MnO₂, CoO, NiO, Cu₂ O, ZnO,Ga₂ O₃, Y₂ O₃, ZrO₂, In₂ O₃, La₂ O₃, CeO₂, and Gd₂ O₃), although nonewere as effective. Compositional ranges for the three major componentswere: PbO (37-60 wt %), Fe₂ O₃ (6-13 wt %), and P₂ O₅ (32-44 wt %),where waste loadings ranged from 14-19 wt %. Practical concentrationranges for glass matrix formation were given to be: PbO (40-66 wt %),Fe₂ O₃ (0-12 wt %), and P₂ O₅ (30-60 wt %). Glasses were melted attemperatures between 900° and 1050° C.

Structure of LIP glass was characterized extensively, using Mossbauerspectroscopy, electron paramagnetic resonance, Raman and infraredspectroscopy, EXAFS (Extended X-ray Absorption Fine Structure),low-angle X-ray scattering, liquid chromatography, and transmissionelectron microscopy. For glasses prepared below 900° C., iron wasincorporated in the glass as Fe⁺³. Average polyphosphate chain lengthwas reduced from more than 15 for iron-free glasses, to about 2.6 forglass with 9 wt % Fe₂ O₃. Thus the improvement in durability was relatedto strengthened cross bonding between polyphosphate chains.

PNL conducted an evaluation of LIP glasses as part of the SecondGeneration HLW Technology Subtask of the Nuclear Waste TreatmentProgram. Their review found LIP glasses to have substantially betterchemical durability than borosilicate glass, although severedevitrification (leading to reduced chemical durability) would result ifglass waste forms were prepared in large canisters. A processing methodwould be required to rapidly cool the material to quench the vitreousstructure. Similarly, LIP glasses were examined for their potentialapplicability for Savannah River Plant (SRP) waste. Phosphate glasseswere found to be highly durable, however, the glass melts were highlycorrosive with existing glass melting equipment. Thermal stability andwaste component solubility were lower than for high-silica glasses. As aresult, borosilicate glasses were found to be, overall, more favorablefor SRP waste treatment.

Ultra low melting temperature lead-tin fluorophosphate glasses wereinvestigated by Tick, P. A. in "Water Durable Glass with Ultra LowMelting Temperatures," Physics and Chemistry of Glass, Vol. 26, No. 6,pp. 149-154 (December 1984) showing glass transition temperatures of 75°C.-150° C. with good resistance to water attack. Reported compositionalranges investigated were 50-61 atomic percent (atomic %) Sn, 3.0-5.7atomic % Pb, and 34-48 atomic % P, with fluorine and oxygen contents of35-74 and 114-149 atoms per 100 cations, respectively. Glass melting wasdone in air at 450° C. Although lead additions had little effect onglass transition temperature, lead-tin phosphate glasses were readilydevitrified without lead as glass component, suggesting that lead has asignificant effect on the bonding character in the glass. With regard tochemical durability, four distinct corrosion behaviors were noted forthe glass formulations tested, corresponding to very high (>10 mg/cm²day) to relatively low (<0.1 mg/cm² day) dissolution rates. Bestdurability was associated with the compositional window defined by8<Sn/Pb<13 and 1<Sn/P<2, with F/Sn=1±0.2. Outside this region, phaseseparation was suspected, producing swelling or accelerated dissolution.There are no reports about their chemical stability in acid or basicsolutions so far.

Few recent advances in phosphate glass waste forms have been reported ascurrent interest seems to be focused on phosphate glass laserdevelopment and associated optical and electronic characterization.Structure models continue to be updated, using high performance liquidchromatography and x-ray spectrometry to analyze glass samples. Chemicaldurability improvements in alkali phosphate glasses were proposed in apatent application by Day and Wilder in "Chemically Durable PhosphateGlasses and a Method for Their Preparation; Patent Application,"Department of Energy, Washington, D.C., PAT-APPL-6-447 847, 1982.Glasses containing 10-60 mole % of Li₂ O, NaO, or K₂ O; 5-40 mole % ofBaO or CaO; 0-10 mole % of Al₂ O₃ ; and 40-70 mole % of P₂ O₅ wereimproved by incorporating up to 23 wt % of nitrogen. Nitrides were thefavored additives.

Other phosphate containing formulations proposed for nuclear wasteimmobilization have been glass ceramics. Certain glass-ceramics orglass-composites may possess higher chemical durabilities than singlephase glasses with respect to dissolution in water or corrosionresistance in harsh environments. Glass-ceramics or glass-composites maybe formulated for vitrification, consisting of a major,thermodynamically stable crystalline phase and a relatively durable,vitreous matrix.

Various combinations of phosphate glass-ceramic matrices have beenmanufactured at research level. For example, new castable glass-ceramicdental materials have the potential for use in many phases ofrestorative dentistry. U.S. Pat. Nos. 3,732,087 and 4,431,420 disclosework which been carried out at Corning Glass Works on glass-ceramicsbased on crystallization of the parent glass. Bio-compatible calciumphosphate glass-ceramics were developed in Japan, possessing extremelyhigh toughness and flexibility. These new materials were described asbeing durable in both acid and alkaline solutions by Abe, Y., et al. in"Calcium Phosphate Glass Ceramics for Biomedical and BiotechnologicalApplications," Asahi Glass Foundation for Industrial Technology,PB91-167056, PC A16/MF A02, (1990). In Canada, glass-ceramics based onpartial crystallization of precursor glasses from the system Na₂ O--Al₂O₃ --CaO--TiO₂ --SiO₂ --X (where X is waste oxides or minor processingadditives) were developed for the possible solidification of fuelrecycle waste was described by Hayward, P. J. in "Review of Progress inthe Development of Sphere-based Glass Ceramics," Scientific Basis forNuclear Waste Management, IX Vol 50, Ed L. O. Werme (1986).Temperatures>1250° C. were required to create the glass; the ceramicphase was then recrystallized during sustained heating at 900°-1050° C.Similar results were obtained in the U.S. for R₂ O--RO--P₂ O₅ and R₂O--Al₂ O₃ --P₂ O₅ glasses (where alkali metals are Na or Li, andalkaline earth metals are Ca or Ba) doped with 1 mol % of TiO₂, ZrO₂, Y₂O₃, La₂ O₃, or Ta₂ O₅. Glass-ceramics from the system SiO₂ --Cs₂ O--Al₂O₃ --(La, Ce)₂ O₃ --P₂ O₅ --Zr were described for high level wasteimmobilization at processing temperatures of <1600° C. in U.S. Pat. No.4,314,909 to Beall, G. H., et al. While these glass-cerarnics exhibitgood chemical durabilities, high melting temperatures and associatedlosses of volatile fission products from the melts would restrictpotential commercial use of these materials.

The glass compositions and vitrification process described above havemany drawbacks. Processes relying on borosilicate glass formulations areincompatible with certain single-shell tank waste components, such asphosphates, calcium oxide and rare earth oxides (Wiemers, et al,PNL-7918 Report, p. 4.5). Moreover, borosilicate compositions requirehigh-temperatures for processing which causes high loss of volatilehazardous metals requiring additional expensive off gas treatment. Whilelead-iron phosphate glasses appear to offer advantages of reducedprocess temperature and improved chemical durability compared withborosilicate glass, their lead content is considerable (40-60 wt %),posing the potential for excessive toxic releases from the glass matrix.

Accordingly, there is still a need in the art of waste disposal for acomposition and method for encapsulation and immobilization ofradioactive, hazardous and mixed wastes.

It is, therefore, an object of the present invention to provide acomposition and process for permanent storage radioactive, hazardous andmixed wastes by encapsulation in low temperature, lead-free phosphateglass compositions. Another object of the present invention is toprovide durable waste forms which can withstand delocalization byecological forces. Another object of the present invention is to developan encapsulating process which does not cause volatilization ofhazardous material.

SUMMARY OF THE INVENTION

The present invention is a new family of lead-free glass compositionsand methods for their use as matrixes to immobilize low level and/orhigh level radioactive wastes in monolithic waste forms. Morespecifically, the present invention provides lead-free glasscompositions for encapsulation of radioactive, hazardous and mixedwastes which include from about 1 mole % to about 6 mole % iron IIIoxide, from about 1 mole % to about 6 mole % aluminum oxide, from about15 mole % to about 20 mole % sodium oxide or potassium oxide, and fromabout 30 mole % to about 60 mole % phosphate. The foregoing compositionshave been prepared at a temperature from about 850° C. to about 950° C.and preferably 900° C. and can be used with or without waste containedtherein.

In another embodiment the lead-free glass compositions may also includeglass modifiers such as, from about 15 mole % to about 20 mole % lithiumoxide, from about 7 mole % to about 20 mole % calcium oxide and fromabout 7 mole % to about 15 mole % magnesium oxide.

Other lead-free glass compositions also provided by the presentinvention for encapsulation of radioactive, hazardous and mixed wastesinclude from about 3 mole % to about 6 mole % sodium oxide, from about20 mole % to about 50 mole % tin oxide, from about 30 mole % to about 70mole % phosphate, from about 3 mole % to about 6 mole % aluminum oxide,from about 3 mole % to about 8 mole % silicon oxide, from about 0.5 mole% to about 2 mole % iron (III) oxide and from about 3 mole % to about 6mole % potassium oxide. This composition may also include up to 1.5 mole% calcium oxide and up to 0.5 mole % magnesium oxide. This lead-freeglass composition have been prepared at a temperature from about 400° C.to about 450° C.

The present invention also provides lead-free glass ceramic compositefor encapsulation of radioactive, hazardous and mixed wastes whichinclude a glass coposition having from about 20 mole % to about 50 mole% tin oxide and from about 30 mole % to about 70 mole % phosphate andfrom about 0.5 weight % to 35 weight % fly ash.

As a result of the present invention, phosphate glass formulations areprovided which are lead-free and may be made at low temperatures. Thelead-free phosphate glass formulations of the present invention can beused to encapsulate and immobilize radioactive, hazardous and mixedwastes. The phosphate glass formulations of the present invention differin type and quantity of network modifiers thereby resulting in glassmatrices with improved chemical durability and reduced meltingtemperatures. The tin-phosphate glass composites of the presentinvention can be used as a host matrix for wastes containing low meltingpoint or easily volatilized components which currently require secondarytreatment under other vitrification processes. The leachability of thenew phosphate glass formulations of the present invention are comparableto lead-iron-phosphate glasses and better than borosilicate nuclearwaste glasses having the important advantage of preventing excessivetoxic lead releases from the glass matrix.

Other improvements which the present invention provides over the priorart will be identified as a result of the following description whichsets forth the preferred embodiments of the present invention. Thedescription is not in any way intended to limit the scope of the presentinvention, but rather only to provide a working example of the presentpreferred embodiments. The scope of the present invention will bepointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates various iron-aluminum-phosphate and tin-phosphateglasses.

FIG. 2 illustrates tin-phosphate glass composites.

FIG. 3 is an X-Ray defraction pattern of iron-aluminum-phosphate glass.

FIG. 4 is an X-Ray detraction pattern of tin-phosphate glass composite.

FIG. 5 is a scanning electron micrograph of glass Fe--Al--P₂ O₅ powderbefore product consistency tests (PCT) having 100 fold magnification.

FIG. 6 is a scanning electron micrograph of the same glass powder as inFIG. 5 taken after product consistency tests having 100 foldmagnification.

FIG. 7 shows leach results for potassium-free iron-aluminum-phosphateglasses of samples BNL 8 (low Fe concentration) BNL 9 (medium Feconcentration), and BNL 5 (high Fe concentration) at 25° C.

FIG. 8 shows a comparison of PCT leach results foriron-aluminum-phosphate glasses taken for BNL 5 at 25° C. and BNL 13 at90° C.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a new family of lead-free phosphate glasscompositions having reduced processing temperatures and improvedchemical durability. Methods for their preparation are also provided.The new glass compositions were developed primarily for use invitrifying radioactive, hazardous and mixed wastes but can be used inother industrial glass applications.

In one aspect of the present invention, the formulation of lead-freeglass compositions is synthesized by admixing from about I mole percentto about 6 mole percent Fe₂ O₃, from about 1 mole percent to about 6mole percent Al₂ O₃, from about 15 mole percent to about 20 mole percentNa₂ O and from about 30 mole percent to about 60 mole percent P₂ O₅. Theresulting admixture is then subjected to temperatures from about 850° C.to about 950° C. and preferably about 90⁰ ° C.

In order to change viscosity, enhance durability and corrosionresistance, modifiers can be added to the glass formulations. Usefulmodifiers include from about 15 mole percent to about 20 mole percentpotassium oxide, from about 15 mole percent to about 20 mole percentlithium oxide, from about 7 mole percent to about 20 mole percentcalcium oxide and from about 7 mole percent to about 15 mole percentmagnesium oxide.

In another aspect of the present invention, the formulation of lead-freeglass compositions also includes tin oxide. More specifically, the glasscompositions are obtained by admixing from about 3 mole percent to about6 mole percent Na₂ O, from about 20 mole percent to about 50 molepercent SnO, from about 30 mole percent to about 70 mole percent P₂ O,from about 3 mole percent to about 6 mole percent Al₂ O₃, from about 3mole percent to about 8 mole percent SiO₂, from about 0.5 mole percentto about 2 mole percent Fe₂ O₃ from about 3 mole percent to about 6 molepercent K₂ O. Optionally, the tin oxide containing glass compositionscan include from about 0 to about 1.5 mole percent CaO, from about 0 toabout 0.5 mole percent MgO. The resulting admixture is then subjected totemperatures from about 400° C. to about 450° C.

In a preferred embodiment of the present invention, a lead-freeglass-ceramic composite is provided by admixing a molten glasscomposition containing from about 2 mole percent to about 10 molepercent sodium oxide, from about 30 mole percent to about 60 molepercent tin oxide and from about 30 mole percent to about 70 molepercent phosphate and up to 35 weight percent fly ash waste whichcontains the remaining oxides listed above.

Once in molten state, the above glass compositions can be used toimmobilize low-level and/or high-level radioactive wastes in monolithicwaste forms. More specifically, the wastes can be added either asliquid, sludge or dry residue to the molten glass.

As used in the present invention radioactive waste refers to low levelradioactive material as defined by Nuclear Regulatory CommissionRegulations ("NRC") set forth in 10 CFR 61. Low level radioactive wastesdo not include spent nuclear fuel, transuranic waste or byproductmaterials which are defined as high-level radioactive wastes in §11 e(2)of the Atomic Energy Act of 1954 at 43 U.S.C. 2014 (e). Low-levelradioactive wastes ("LLW") include radioactive material found inevaporator concentrate, ion exchange resins, incinerator bottom ash,filtration sludges, and contaminated filters and membranes. Cs-137,Co-57 and Sb-125 are common radioactive constituents.

Hazardous waste refers to solid waste that may pose a substantialpresent or potential hazard to human health and the environment whenimproperly treated, stored, transported, disposed or otherwise managedas defined by §1004 (5) of the Resource Conservation and Recovery Act asset forth at 40 CFR 261, EPA 55 Fed. Reg. 11862 (March 1990). Hazardouswaste does not include low level radioactive waste. Hazardous materialsinclude arsenic, lead, cadmium, mercury and other metals identified byEPA as toxic metals.

Mixed waste as used herein refers to waste which includes low levelradioactive waste and hazardous waste. A more complete definition ofmixed waste is set forth in a Memorandum of Understanding between theNRC and EPA and as published at 51 Fed. Reg. 24505 (Jul. 3, 1986). Mixedwastes include aqueous process streams, sludges, debris, and ash waste.

Waste types which may be encapsulated according to the encapsulationprocess of the present invention include incinerator blowdown solution,sludges, molten salt oxidation residues, ion exchange resins andmunicipal solid waste incinerator ash. Incinerator blowdown solution mayinclude chloride salts, incinerator fly ash, toxic metals andradionuclides. Sludges treatable according to the process of the presentinvention include calcium and sodium carbonate, nitrates, EDTA chelatingagent and toxic metals. Molten salt oxidation process residues includeprimarily calcium carbonate salts and miscellaneous fission products andtoxic metals. Ion exchange resins are used to remove hazardouscontaminants from process streams and can contain radionuclides andtoxic metals.

The focus of the examples set forth below has been to provide cruciblestudies of the novel lead-free glass compositions and composites of thepresent invention. Two types of phosphate glass formulations wereprepared: one based on iron-aluminum-phosphate system and the otherbased on tin-phosphate system. Modifying cations were added to eachglass in order to improve chemical durability, stability and/orprocessability of the glass matrices. For example, modifying cationsinclude oxides of lithium, sodium, potassium, calcium, and magnesiumadded or substituted in varying proportions to theiron-aluminum-phosphate glass. Silicon, sodium, potassium, iron andaluminum were used to modify the tin-phosphate system. The resultingglass compositions and composites were evaluated by using leaching testsand product consistency tests. Product consistency tests (PCT) arestandard glass leaching tests developed by Westinghouse Savannah RiverCompany, presently submitted as ASTM standard C26. 13 as morespecifically discussed by Jantzen, C. M., et al., in "Standard TestMethods for Determining Chemical Durability of Nuclear Waste Glassed:the Product Consistency Test (PCT)," version 6.0, Westinghouse SavannahRiver Co., Aiken, S.C., (April, 1993). In addition, x-ray defractionanalysis (XRD) and scanning electromicroscopy (SEM) have been conductedto analyze the composition of the lead-free glasses and composites ofthe present invention.

EXAMPLES

The examples set forth below also serve to provide further appreciationof the invention but are not meant in any way to restrict the effectivescope of the invention.

Example 1

In this example, glasses were prepared in small batches of reagent-gradeoxide powders. The batches were from about 20 grams to about 50 grams.Raw chemicals were premixed in a rotating ball mill and then chargedinto 50 cc porcelain crucibles. The premixed powders were melted in alaboratory convection oven under atmospheric conditions. The resultingiron-aluminum-sodium-phosphate glasses had the following composition:1-6 mole % Fe₂ O₃ ; 1-6 mole % Al₂ O₃, 15-20 mole % Na₂ O; and 30-60mole percent P₂ O₅. The glasses were then melted at temperatures of 900°C. from about 2 hours to about 4 hours.

Tin-phosphate glasses were prepared in the same manner and they had thefollowing composition: 3-6 mole % Na₂ O; 20-50 mole % SnO; 30-70 mole %P₂ O₅ ; 3-6 mole % Al₂ O₃ ; 3-8 mole % SiO₂ ; 0.5-2mole % Fe₂ O₃ ; and3-6 mole % K₂ O. Tin-phosphate glasses were melted at 450° C. for fromabout 1 to about 2 hours. The resulting molten glasses were cast instainless steel molds and allowed to cool to room temperature. Glassbuttons were obtained which were, for the most part, clear with fewseeds or air bubbles. The samples were not stirred during melting. Asshown in FIG. 1 attached, colors ranged from light to dark brown for theiron-aluminum-sodium-phosphate glasses. The above samples were annealedat 150° C. for up to one month and showed no evidence ofdevitrification. These samples provide an excellent encapsulation mediumfor radioactive, hazardous and mixed wastes.

Example 2

In this example, tin-phosphate glass composites containing additiveswere prepared in order to determine the capabilities of the lowtemperature glass as an encapsulation medium. The tin-phosphate glasscomposites were prepared in accordance with the procedure discussed inExample 1 above. The composites had the following formulation: 2-10 mole% Na₂ O; 3-6 mole % Al₂ O₃ ; 3-8 mole % SiO₂ ; 0.5-2 mole % Fe₂ O₃ ; 3-6mole % K₂ O; 30-60 mole % SnO; and 30-70 mole % P₂ O₅. Sodium, tin andphosphate oxides in the proportions shown above were mixed with 35weight % fly ash waste which contained the remaining oxides listedabove. The resulting glass-ceramics are shown in FIG. 2 attached. Thecolors ranged from light to dark gray and they were similar inappearance to glassy slags.

FIGS. 3 and 4 show XRD patterns typical of the synthesized glass andglass-ceramic materials. While the XRD patterns foriron-aluminum-phosphate glass were representative of amorphousmaterials, those taken from powdered tin-phosphate glass compositeindicated the presence of crystalline fly ash waste bonded orencapsulated within the glass matrix. These samples have been preparedat low temperatures of about 400° C.-450° C. and have provided anexcellent medium for incorporation of fly ash.

Example 3

In this example, glass samples obtained in Examples 1 and 2 werecomminuted to the -100 to +200 mesh size range as required for the PCTprocedure desribed in Jantzen, C. M., et al., supra at pp 8-9. Largepieces were crushed inside a polyethylene bag by carefully striking witha hammer. Fines were further ground using a porcelain mortar and pestle.Powders were sieved through stainless steel sieves. Screens were cleanedand dried between batches in order to prevent cross contamination of theglasses. The density of glass samples was measured by using Quantachromepycnometer. Density data is required to calculate glass surface area forthe PCT procedure. The samples were measured both as chunks or buttonsand also as crushed glass powders. Surface areas for each glass werecalculated by using the equation in Appendix I of the PCT test method asdiscussed in Jantzen, C. M., et al. supra.

PCT testing was carried out by using teflon perfluoralkoxy vessels.Because the intended use for the glass was as host with mixed ratherthan high-level radioactive waste, baseline leach tests were conductedat ambient temperature according to test method B rather than at 90° C.as specified by method A for high-level radioactive waste forms.However, in order to provide a comparison with baseline borosilicateleach test results, two leach tests were conducted at 90° C. oniron-aluminum-phosphate glasses. These glasses had the followingcomposition: BNL-13 contained about 15 mole % Na₂ O, about 10 mole % Li₂O, about 12 mole % CaO, about 12 mole % MgO, about 4 mole % Fe₂ O₃,about 3 mole % Al₂ O, and about 44 mole % P₂ O₅ ; BNL-14 contained about15 mole % Na₂ O, about 10 mole % K₂ O, about 12 mole % CaO, about 12mole % MgO about 3.5 mole % Fe₂ O₃, about 3 mole % Al₂ O₃ and about 44.5mole % P₂ O₅. The leach tests were conducted in purified water (ASTMType I) in a volume which had 10 times the mass of the glass sample fora period of about 7 days. Weights of the glass samples tested range fromabout 0.38 to about 1.17 grams. All tests were done in triplicate. Oncompletion of the leach test, leachates were analyzed by inductivelycoupled plasma (ICP) spectroscopy for matrix cations. The chemistry ofthe parent glasses were determined by totally digesting the referenceglasses in aqua regia and hydrochloric acid and analyzing by ICP formatrix components.

Leach rates of the dissolved metals were calculated as mass of metalleach (per gram of glass sample) divided by surface area (per gram ofglass) and leach time. The data was normalized by dividing the originalamount of each element present in the glass, as determined by digestionof the parent glass. The leach data for all iron-aluminum-phosphateglasses is shown in Table 1 below. The leach data for tin-phosphateglass composites is shown in Table 2 below.

                                      TABLE 1    __________________________________________________________________________    PCT Leach Results for Fe.sub.2 O.sub.3Al.sub.2 O.sub.3P.sub.2 O.sub.5    Glasses.    __________________________________________________________________________     ##STR1##    __________________________________________________________________________     .sup.a Leach data are the mean of 3 replicates; standard deviation shown     in parenthesis.     .sup.b NA = not applicable (this metal not included in this formulation).     .sup.c Leach test conducted at 90° C.

                                      TABLE 2    __________________________________________________________________________    PCT Leach Results for Sn--P.sub.2 O.sub.5 Glass-Composites.    Normalized Leach Rate, g/m.sup.2 /d.sup.a    Na       K    Fe   Al   Si   Sn   P    __________________________________________________________________________    BNL-6        3.93E-02             NA.sup.b                  1.99E-02                       7.33E-03                            ND.sup.c                                 6.78E-02                                      8.63E-02        (4.74E-03)                  (1.35E-03)                       (2.74E-04)                                 (5.51E-03)                                      (7.58E-03)    BNL-12        NA   7.28E-02                  1.75E-02                       1.46E-02                            3.47E-03                                 9.59E-02                                      9.00E-02             (9.09E-04)                  (8.23E-05)                       (3.19E-04)                            (5.66E-05)                                 (1.24E-03)                                      (2.02E-04)    __________________________________________________________________________     .sup.a Leach data are the mean of 3 replicates; standard deviation shown     in parenthesis.     .sup.b NA = not applicable (this metal not included in this formulation).     .sup.c ND = not detected.

Mean and standard deviation, as shown in parenthesis, of three replicatetests are reported in the tables for each glass composition. SamplesBNL-5, BNL-8, BNL-9 and BNL-13 are similar in that the glasses eachcontain about 15 mole % Na₂ O, about 10 mole % Li₂ O, about 12 mole %CaO, and about 12 mole % MgO, with variable amounts of Fe₂ O₃ from about1.5% mole to about 4 mole %, Al₂ O₃ from about 1.5% mole to about 3 mole%, and P₂ O₅ from about 44 to about 48 mole %. Samples BNL-10, BNL-11,and BNL-14 each contain about 15 mole % K₂ O, about 10 mole % Li₂ O,about 12 mole % CaO, and about 12 mole % MgO, with variable amounts ofFe₂ O₃ from about 1.5 mole % to about 4 mole %, Al₂ O₃ from about 1.5mole % to about 3 mole %, and P₂ O, from about 44% mole to about 48mole%. Samples BNL-6 and BNL-12 are composites of approximately 35 wt %flyash encapsulated in a tin-phosphate glass matrix. These compositesnormally contain from about 2 mole % to about 10 mole %, NaO, from about3 mole % to about 6 mole % K₂ O, from about 3 mole % to about 8 mole %SiO₂, from about 0.5 mole % to about 2 mole % Fe₂ O₃, from about 3 mole% to about 6 mole % Al₂ O₃, from about 30 mole % to about 60 mole % SnO,and from about 30 mole % to about 70 mole % P₂ O although compositioncan vary depending on composition and quantity of flyash encapsulated inthe composite. The glasses set forth in the tables have been arrangedwith increasing modifier (Fe/Al) content. Leach test blanks, i.e.,containing no glass samples were included with each series of tests. Theblanks showed no detectable metals present after a 7-day period. Theabove results indicate that PCT leachability of glass matrix componentscan be improved by up to a factor of 10 through proper selection ofcation modifiers.

SEM microscopy was conducted on post-leach test glass powders of samplesBNL 13, BNL 14 to look for evidence of corrosion. As shown in FIGS. 5and 6, the leached samples were indistinguishable from the untestedsamples. The corrosion rate was reduced with increasing additions of Fe,as depicted in FIG. 7. The PCT leach results conducted at 90° C. werevirtually identical to tests conducted at room temperature. A directcomparison can be made between the leach test results for sample BNL- 13which were taken at 90° C. and those conducted on BNL-5 at roomtemperature. As previously noted, BNL-5 and BNL-13 have the samecomposition, namely about 15 mole % Na₂ O, about 10 mole % Li₂ O, about12 mole % CaO, about 12 mole % MgO, from about 15 to about 4 mole % F₂O₃, from about 1.5 to about 3 mole % Al₂ O₃, about 44 mole % P₂ O₅. Ametal-by-metal comparison for these tests is shown in FIG. 8. The higherleach temperature yielded slightly higher releases for alkali metals andphosphorous, and slightly lower releases for the other metals present.Although formulations have not been optimized, the lowest leachabilityof the formulations tested was achieved in glasses containing sodium, asopposed to those containing potassium. Alkali and alkaline earth metalsmade up a significant fraction of these glasses, indicating theircapacity to incorporate and retain significant salt contaminants likelyto be found in the Tank Waste Remediation System (TWRS), a permanentstorage of nuclear waste facility found at Hanford, Wash. State.

Leach rates for tin-phosphate composites were only slightly higher thanthose obtained for the best Fe₂ O₃ --Al₂ O₃ --P₂ O formulations. XRDdata on these composites indicates that the glassy matrix was retained.Chemical constituents in the dry surrogate waste, namely Ca, Al, and Si,were compatible with the low temperature glass.

PCT data for both iron-aluminum-phosphate and tin-phosphate matricescompared favorably with lead-iron-phosphate glasses and referenceborosilicate high-level waste glasses. Sales and Boatner quote leachrates of 0.73-1.25, 0.93-2.30, and 0.99-2.00 g/m² /d, for Si, B, and Na,respectively, from borosilicate defense and commercial waste glasses in"Physical and Chemical Characteristics of Lead-Iron Phosphate NuclearWaste Glasses," Journal of Non-Crystalline Solids, vol. 79, pp 84-116(1986). Comparable leach results from lead-iron-phosphate waste glasseswere <0.006, <0.001, <0.02, and <0.02 g/m² /d, for P, Fe, Al and Na.These results are for leach tests done in distilled water at pH 7, 90°C. for 30 days.

Thus, while there have been described what are presently believed to bethe preferred embodiments of the present invention, those skilled in theart will appreciate that other and further modifications can be madewithout departing from the true scope of the invention, and it isintended to include all such modifications and changes as come withinthe scope of the claims as appended herein.

What is claimed is:
 1. A lead-free glass composition for encapsulationof radioactive, hazardous and mixed wastes which comprises from about 3mole % to about 6 mole % sodium oxide, from about 20 mole % to about 50mole % tin oxide, from about 30 mole % to about 70 mole % phosphate,from about 3 mole % to about 6 mole % aluminum oxide, from about 3 mole% to about 8 mole % silicon oxide, from about 0.5 mole % to about 2 mole% iron (III) oxide and from about 3 mole % to about 6 mole % potassiumoxide.
 2. The lead-free glass composition of claim 1, further comprisingup to 1.5 mole % calcium oxide and up to 0.5 mole % magnesium oxide. 3.The lead-free glass composition of claim 1, having a melt temperaturefrom about 400° C. to about 450° C.
 4. A lead-free glass-ceramiccomposite for encapsulation of radioactive, hazardous and mixed wasteswhich comprises a glass composition comprising from about 20 mole % toabout 50 mole % tin-oxide and from about 30 mole % to about 70 mole %phosphate and from about 0.5 weight % to 35 weight % fly ash.
 5. Thelead-free glass-ceramic composite of claim 4, further comprising fromabout 2 mole % to about 10 mole % sodium oxide.
 6. The lead-freeglass-ceramic composite composition of claim 4, wherein said fly ashcomprises from about 3 mole % to about 6 mole % sodium oxide, from about3 mole % to about 6 mole % aluminum oxide, from about 3 mole % to about8 mole % silicon oxide, from about 0.5 mole % to about 2 mole % iron(III) oxide, from about 3 mole % to about 6 mole % potassium oxide, upto 1.5 mole % calcium oxide and up to 0.5 mole % magnesium oxide.